Sulphide ores containing metals such as copper, gold, platinum group metals, nickel, lead and zinc are recovered commercially by fine grinding and flotation to concentrate the valuable component and discard the gangue.
The conventional process involves grade control drilling to delineate the ore, blasting the necessary waste (below economic cut-off-grade (CoG)) and ore, loading trucks to haul the ore for primary crushing and the waste to a disposal area. The crushed ore is conveyed to a milling process, typically using semi-autogenous grinding (SAG) or high pressure grinding rolls (HPGR); followed by ball milling to fully liberate the valuable particles at a p80 of around 75-200 micron. Then the ore is separated using a flotation process to produce a saleable concentrate and tailings. The tailings from flotation are pumped to a tailings storage facility (TSF) and stored in perpetuity.
As the conventional process chain requires all of the ore to be ground finely, it consumes large quantities of energy (typically 20 kwh/t ore) and water (0.5-1.0 tonne water per tonne of ore). The majority of this water is lost in the fine tailings, where it is intimately mixed and retained with the very fine residue produced from the conventional flotation process.
The shortage of available water in some locations has generated different approaches to water saving. Some mines have installed desalination plants on the adjacent coast, and pumped the desalinated water to the processing facility. Others have installed large filter presses to squeeze as much water from tailings as possible. However these solutions both suffer very high capital and operating costs. The high cost of fine grinding and high water consumption also means the recovery efficiency of the in-ground resource is limited to that which is economic to process.
At any particular time in the mine life, the CoG for ore is set to maximise the feed grade and hence production from the available processing capacity. This CoG may be variously constrained by available water, or tailings capacity, or the installed processing capacity. Whichever constraint applies, economically attractive ore is often being rejected to a waste pile, simply because higher grade materials are available at the time. Even if a low grade stockpile is introduced to manage the material which is above economic processing grade but below the CoG of the day, the materials handling cost of stockpiling and reclaiming this marginally attractive material later in the mine life, implies a fraction of the economic resource will be lost to the waste rock pile.
If the run-of-mine ore could be beneficiated prior to fine grinding, to reject as waste material which is below economic cut-off-grade, with a high recovery of the values and a reasonably high upgrade ratio, and in a relatively low cost operation, the unit costs and consumption of water would be reduced. The consequential grade of feed to processing would be increased. And the grade of any stockpile required by a constraint to available processing capacity would be higher, resulting in improved margins when eventually recovered.
Recognising the value associated with removing a fraction of the ore that is below CoG, and preferably below economic treatment grade, various beneficiation techniques have been proposed. For sulphides, these are usually based around gravity techniques such as dense media separation, spirals, etc., and rock sorting methods. But for most sulphide ores, these beneficiation techniques fail either the upgrade ratio/recovery or cost hurdles for implementation.
If the beneficiation parameters are set to reject sufficient ore (i.e. achieve a high upgrade ratio) to economically warrant the cost of the beneficiation process, the loss of values is excessive. This means an increase in mining cost per tonne of product, and a decrease in the effective utilisation of the overall resource.
Hence run of mine (RoM) ores are conventionally ground to very fine sizes to achieve complete liberation of the valuable components then floated, despite the obviously high cost of comminution and water consumption.
Recently, the ability to use a chemically based coarse flotation process for beneficiating sulphides, using a fit for purpose flotation cell has been proposed by Eriez Flotation Division (EFD), a wholly owned subsidiary of Eriez Manufacturing Co. Using this coarse flotation technology, the ability to dry stack sand residue was recognised, thus opening up another potential beneficiation technique to reduce water and energy (WO2016/170437). As a one off process for water recovery, it is very useful, but due to particle size vs. recovery constraints on coarse flotation, and the size separation precision of hydrocyclones, only 30-50% of the ore ends up as sand. Hence water consumptions and tailings volumes are typically only reduced by some 25-40%.
In a second beneficiation technique for sulphidic ores, the differential fracture along the mineralised grain boundaries, causing most of the sulphides to concentrate in the finer size ranges, has been recognised. The differential fracture enables screening to reject the coarsest rocks, which usually contain the lowest grade. This technique was first introduced in Bougainville in the late 1980s (Australasian Institute of Mining and Metallurgy, Papua New Guinea Mineral Development Symposium, 27-28 Jun. 1986, Madang, The Application or Preconcentration by Screening at Bougainville Copper Limited, Burns R S and Grimes A W, the content of which is incorporated herein by reference). The beneficiation technique is being actively re-examined by a number of operations under the CRC Ore trademark of ‘Grade Engineering’. CRC ORE is a not for profit organisation funded by the Australian Federal Government and the global minerals industry http://www.crcore.org.au/main/index.php/solutions/grade-engineering.
And finally, beneficiating using bulk sorting has also been proposed. The development of sensors that can adequately determine average grades on a conveyor belt or shovel at a high rate, allows for the stream of broken rock to be identified and diverted to either ore or waste. Reference: Valery et. al. World Mining Congress 2016; Minesense http://www.minesense.com/products:
The ShovelSense™ shovel product is a real-time mineral telemetry and decision support system for surface or underground applications. It is a retrofit package installed in the dipper of surface shovels or into the scoop of underground machines such as scooptrams or LHD's. The ShovelSense™ platform is used for:                Measurement of ore quality while material is being scooped into the dipper;        Reporting of ore quality and type to the grade control/ore routing system        Real-time, online decision support for ore/waste dispatch decisions.        
Bulk sorting takes advantage of the natural heterogeneity of orebodies, with the separation of zones of high and low grade material that would conventionally be mixed into homogenised run-of-mine ore. The weakness of bulk sorting is it can only reject those zones that are low grade at the time of sensing, and hence to retain an acceptable upgrade ratio it must be installed prior to significant homogenisation of the ore.
Despite these three recent and quite distinct beneficiation techniques being relatively well known, none has yet found widespread use in the mining industry. This may be at least partially attributed to the same upgrade ratio, recovery, and cost reasons that have hampered the implementation of traditional gravity based beneficiation.
In summary, the mining industry is very capital intensive, a large consumer of water and energy, and only partially recovers the values contained in the earth that is mined. Whilst beneficiation techniques are known which can potentially address these issues, they have been considered in isolation to resolve each constraint individually, and mostly found to be uneconomic.